Breakdown of ¿Esos guantes son los suyos?
ser
to be
esos
those (masculine)
suyo
his / hers / theirs / yours (formal)
el guante
glove
Questions & Answers about ¿Esos guantes son los suyos?
Does los suyos match the owners ("them") or the object being owned ("gloves")?
It matches the object being owned! Even though the gloves belong to "them" in this translation, los suyos is masculine and plural because it replaces los guantes (masculine, plural). In Spanish, possessive pronouns always match the thing owned, never the owner.
Since los suyos can mean his, hers, yours, or theirs, how do we know it means "theirs" here?
We only know because of the context! The pronoun suyo (and its forms like los suyos) is very flexible and can refer to anyone you would call usted, él, ella, ustedes, ellos, or ellas. If this sentence popped up out of nowhere, it could equally mean "Are those gloves his?" or "Are those gloves yours?"
Could I drop the los and just say ¿Esos guantes son suyos?
Yes, you can! When a possessive pronoun comes right after the verb ser (like son here), it is very common to drop the article. ¿Esos guantes son suyos? means the exact same thing and sounds completely natural. Keeping the los just adds a slight emphasis, like asking "Are those gloves the ones that are theirs?"
Why is it esos guantes and not estos guantes?